The present invention relates to a process for obtaining at least one very pure tertiary olefin by decomposing the corresponding ether in the presence of at least one silica-based catalyst optionally in the absence of water vapor.
The tertiary olefins obtained according to the present invention are in accordance with the formula ##STR1## and are prepared from the corresponding ethers of formula ##STR2## in which R and R.sub.1, which can be the same or different, are in each case chosen from the group formed by the hydrogen atom, alkyl, arylalkyl, aryl and alkyl aryl radicals and R.sub.2 and R.sub.3, which can be the same or different, are each chosen from within the group formed by alkyl, aryl alkyl, aryl and alkyl aryl radicals.
Tertiary olefins constitute a very important starting material for the preparation of certain chemical products and in particular polymers, so that it is essential to have very pure tertiary olefins.
It is known that by reacting an olefin or a mixture of olefins with a primary alcohol in the presence of an acid such as sulphuric acid, or a solid having an appropriate acidity, it is possible to obtain the corresponding ether or ethers. Moreover, the speed of said reaction is dependent on the operating conditions and also the nature of the radicals R, R.sub.1, R.sub.2 and R.sub.3. It is possible to selectively react in an olefin fraction, such as a fraction obtained from thermal or catalytic cracking, tertiary olefins with at least one primary alcohol. The then formed tert.-alkyl-alkyl-ether can easily be separated from the olefins which have not reacted and then decomposed in order to again give the corresponding purified tertiary olefin.
The prior art describes catalytic processes for the production of tertiary olefins by the catalytic decomposition of the corresponding ethers. However, as this reaction is aided at high temperature, certain prior art catalysts lead to the formation of dialkyl ethers and water from the primary alcohol and therefore to a varying alcohol loss, which is prejudicial to the economics of a process in which the alcohol is normally recycled to the synthesis section of the tert.-alkyl-alkyl-ether.
Another undesired side reaction aided by certain prior art catalysts is the formation of oligomers from the olefin and which in particular lead to a significant drop in the purity of the olefin produced.
There has already been proposed a process for obtaining an olefin from the corresponding ether, which largely obviates the aforementioned disadvantages (U.S. Pat. No. 4,395,580). This process consists of operating with the aid of at least one catalyst having optimized acid characteristics and in the presence of water vapor. This addition of water vapor makes it possible to increase the alcohol and olefin yields, but complicates the diagram of the process scheme and leads to significant extra costs.
The decomposition of tert. alkyl ethers on a silica-doped alumina-based catalyst is known (U.S. Pat. No. 4,006,198). However, this catalyst leads to the formation of dialkyl ethers on raising the reaction temperature.
It has already been proposed to prepare tertiary olefins by decomposing corresponding ethers in the presence of silica-based catalysts doped by alumina or an oxide of an element chosen from among chromium, beryllium, titanium, vanadium, manganese, iron, cobalt, zinc, zirconium, rhodium, silver, tin, antimony and boron (U.S. Pat. No. 4,254,296). However, as indicated by the authors of U.S. Pat. No. 4,254,296 in patent application WO 87/00166, these catalysts are difficult to prepare and have high production costs. In addition, they have a relatively short life, requiring an increase in the reaction temperature, whilst leading to the formation of undesired products.
Patent application WO 87/00166 describes improved catalysts for this application, constituted by silica modified by the addition of 0.1 to 1.5% by weight of alumina based on the silica. Although these catalysts are more stable than those previously referred to, they still suffer from a performance deterioration. Maintaining conversion above 70% makes it necessary to increase the temperature to 130.degree. to 350.degree. C. and, following 3000 hours operation, it is necessary to operate at temperatures of 250.degree. C. in order to maintain said conversion, which leads to a methanol yield drop to below 99%.